A grain-oriented electromagnetic steel sheet contains easy magnetization axes oriented in a rolling direction (hereinafter also referred to L-direction) in a manufacturing process and has remarkably low iron losses in the L-direction. In manufacturing the grain-oriented electromagnetic steel sheet, when the steel sheet is irradiated with a laser beam in the direction substantially perpendicular to the L-direction, the iron losses in the L-direction are further reduced. The grain-oriented electromagnetic steel sheet is used mainly as a material for an iron core of a large-sized transformer which has severe requirements for iron losses.
FIG. 8 is a schematic diagram illustrating a conventional method for irradiating a surface of a grain-oriented electromagnetic steel sheet with a laser beam. FIG. 5A is a schematic diagram illustrating a method for manufacturing an iron core of an ordinary transformer and FIG. 5B is a schematic diagram illustrating the iron core.
As illustrated in FIG. 8, in manufacturing a grain-oriented electromagnetic steel sheet whose magnetic domains are controlled by laser beam irradiation, the grain-oriented electromagnetic steel sheet 12 is irradiated with a laser beam while laser beam scanning is being performed at a velocity of Vc in substantially parallel to the plate width direction (hereinafter referred to as C-direction). The C-direction is orthogonal to the L-direction. Besides, the grain-oriented electromagnetic steel sheet 12 is conveyed at a velocity of VL in the L-direction. Thus, a plurality of laser beam irradiation portions 17 extending in substantially parallel to the C-direction aligns at constant intervals of PL. In manufacturing an iron core 4 of a transformer, as illustrated in FIGS. 5A and 5B, the grain-oriented electromagnetic steel sheet is sheared so that a magnetization direction M of an iron core element 3 constituting the iron core 4 and the L-direction meet each other, and the iron core elements 3 obtained by the shearing are layered.
In the iron core 4 manufactured in this way, the L-direction and the magnetization direction M meet each other at most portions thereof. Accordingly, the iron losses of the iron core 4 are in approximate proportion to the L-direction iron losses of the grain-oriented electromagnetic steel sheet of a raw material.
On the other hand, at joint portions 5 between the iron core elements 3 of the iron core 4, the L-direction and the magnetization direction M shift from each other. Accordingly, the iron losses of the joint portions 5 are different from the L-direction iron losses of the grain-oriented electromagnetic steel sheet of a raw material and are affected by iron losses in the C-direction. Thus, a region 6 having high iron losses exists. Particularly, in the iron core using the grain-oriented electromagnetic steel sheet whose L-direction iron losses are significantly reduced by laser beam irradiation, an effect of the C-direction iron losses becomes relatively larger.
Transformers are used at a large number of positions of power transmission equipment from a power plant to power consumption locations. Accordingly, when iron loss per transformer changes by even about 1%, power transmission loss significantly changes at the whole power transmission equipment. Consequently, there is strongly demanded a method for manufacturing a grain-oriented electromagnetic steel sheet capable of reducing C-direction iron losses while L-direction iron losses are being restrained to be low by laser beam irradiation.
However, a mechanism for improving C-direction iron losses has not been clarified nor a method for reducing iron losses in the two directions of L-direction and C-direction has been established until now.
In a conventional method for improving iron losses of a magnetic steel sheet, a principal objective is to reduce L-direction iron losses. For example, Patent Document 5 discloses a method for manufacturing a grain-oriented electromagnetic steel sheet which is irradiated with a laser beam by defining a mode of a laser beam, a light condensing diameter, power, a laser beam scanning velocity, an irradiation pitch and the like. However, there is no description of C-direction iron losses.
In addition, a method in which attention is focused to improvement of the iron losses in the C-direction has also been proposed.
Patent Document 1 discloses a method for irradiating a laser beam in parallel to an L-direction. However, this method reduces iron losses in the C-direction, but does not reduce iron losses in the L-direction. Since an effect of the L-direction iron losses is large as described above, iron loss of a transformer becomes larger than that of the grain-oriented electromagnetic steel sheet with improved iron losses in the L-direction by irradiating a laser beam perpendicular to the L-direction.
Patent Document 2 discloses a method for irradiating a laser beam in parallel to two directions of L-direction and C-direction. However, this method, irradiating a laser beam twice, complicates a manufacturing process and lowers production efficiency by at least one-half.
Patent Documents 3 and 4 disclose a method for irradiating a laser beam while an irradiation direction and an irradiation condition are being changed for each cut element after a grain-oriented electromagnetic steel sheet not subjected to laser beam irradiation is sheared into a desired shape, in manufacturing an iron core. However, in an iron core manufactured according to this method, a portion in which only the iron losses in the L-direction are improved and a portion in which only the iron losses in the C-direction are improved are mixed, therefore it cannot be said that significantly good iron losses are obtained. Besides, to improve iron losses in two directions of the L-direction and C-direction, it is necessary to change conditions and irradiate a laser beam twice. Further, there is a problem of very low productivity because the grain-oriented electromagnetic steel sheet is irradiated with a laser beam for each element after the grain-oriented electromagnetic steel sheet is sheared.
Patent Document 1: Japanese Laid-open Patent Publication No. 56-51522    Patent Document 2: Japanese Laid-open Patent Publication No. 56-105454    Patent Document 3: Japanese Laid-Open Patent Publication No. 56-83012    Patent Document 4: Japanese Laid-Open Patent Publication No. 56-105426    Patent Document 5: International Publication Pamphlet No. WO 04/083465